1. Field of the Invention
This invention relates generally to sensors, and, more particularly, to sensors of various analytes fabricated by positioning on a fiber field access block a measurand sensitive waveguide whose properties are changed as a result of interactions with the environment.
2. Description of the Related Art
One conventional device for detecting the presence of analytes is an extrinsic polymer swelling sensor that uses reflections from a surface which is moved by the swelling of the polymer to provide an indication of the concentration of analytes. This type of sensor cannot be protected from the effects of temperature and is vulnerable to vibrations, pressure changes, and contaminants that may exist in the analyte.
Several other fiber optic chemical sensors rely on conventional spectroscopic instrumentation. In these cases the detector is usually a photomultiplier tube or a photodiode. The sources are usually a tungsten-halogen lamp, a xenon arc lamp, or an argon ion laser. These types of sensors require sophisticated, miniaturized demodulation equipment.
Other attempts have been made to use sensors to exploit properties of fiber optic media to transport light between a sample and the light source or detector. These techniques use light wavelengths in the blue or ultraviolet range in order to be sensitive to the spectral characteristics of the species formed by reaction of an indicator reagent with the analyte. These wavelengths require special lasers and are not transmitted well due to the absorption properties of the fibers. Additionally, the sensor stability is generally limited by the indicator. Since the detection mechanism requires photoexcitation, photodecomposition becomes an additional problem. Moreover, the associated dyes are usually unstable.
The use of a thin film as an optical waveguide is described in P. K. Tien, "Light Waves in Thin Films in Integrated Optics," Applied Optics, November 1971, vol. 10, no. 11, pp. 2395-2413, which is herein incorporated by reference. For a thin film to support propagating modes and act as a waveguide, its refractive index n.sub.r must be larger than the refractive indices of the material above and below it. The resonance condition in a rectangular waveguide is given by: EQU 2kn.sub.r tcos.theta..sub.r -2.phi..sub.ru -2.phi..sub.rb =2m.pi.
where k is the angular frequency of the light wave in the rectangular waveguide divided by the speed of light in a vacuum; t is the thickness of the waveguide; .theta..sub.r is the angle between the light path and the normal of the rectangular waveguide; 2.phi..sub.ru is the phase change the wave suffers due to the total reflection at the upper film boundary and 2.phi..sub.rb is the phase change the wave suffers due to the total reflection at the bottom film boundary (representing the Goos-Haenchen shifts); and m is an integer (0, 1, 2, 3, . . . ) representing the order of the mode. Thus the response changes with the thickness of the waveguide and the index of refraction.
The use of a thin film over a polished fiber as a filter is described in C. A. Millar, M. C. Brierley, & S. R. Mallinson, "Exposed-core single-mode-fiber channel-dropping filter using a high-index overlay waveguide," Optics Letters, April 1987, vol. 12, no. 4, pp. 284-86. The waveguide couples out light when its phase velocity matches the phase velocity of the fiber and the interaction length of the fiber-waveguide coupler equals the coupling length. An approximate calculation of the wavelength which will be coupled out is provided by the equation: EQU .lambda..sub.m =(2t/m)(n.sub.r.sup.2 -n.sub.ef.sup.2).sup.1/2
where n.sub.ef is the fiber mode index of refraction.